Spyke
sopuli.xyz

If I'm not mistaken, the foss fork of signal (molly) does let you link 2 android devices

Edit: Nevermind

4

Molly can also work along with Signal, if you don't want to loose any history in your primary insall, you can still install Molly on your secondary device and link it to Signal

2

Yea this and two-phone support is the main stuff I'm still waiting for

2

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Android tablet as a linked device | Spyke